As conventional optical memories, the CD (Compact Disc), DVD (Digital Video Disc or Digital Versatile Disc), HD-DVD (High-Definition Digital Versatile Disc) and BD (Blu-Ray Disc™) are known. For these optical memories, the same method is used for recording and reproducing optical information. In response to demands for high density of recorded information and large capacity of the memories, mainly the wavelength of the used laser has been shortened, and/or a numerical aperture of a lens that focuses light on the recording medium has been increased. However, it is difficult to increase the numerical aperture of a lens more than the numerical aperture used for the BD. Moreover, if the wavelength of the laser is further shortened, absorption of the laser light in the air cannot be ignored, thereby reducing the usage efficiency of the laser light. Furthermore, there is a problem that such a device would not be practical as special peripheral optical elements would be required. Therefore, to materialize optical memories with higher density and larger capacity than the BD, new optical information recording/reproduction technologies are required.
A hologram memory is expected as an optical memory with a capacity larger than the BD. The hologram memory is an optical memory that utilizes “holographic recording (i.e., means for recording information on a recording medium by holography),” in which laser light is separated into two rays that are overlapped with each other on a recording medium after information to be recorded (recording information) is provided in one of the two rays of the laser light. More specifically, the holographic recording is a technique to record information on a recording medium by spatially separating the laser light into two rays, and by having a signal wave, which is one of the two rays of the laser light provided with information, interfere with a reference wave, which is the other ray of the laser light, in the recording medium.
This holographic recording is also expected as a terabyte-class large-capacity recording technology in recent years. In normal photographic technologies, only light intensity (brightness) can be recorded as information. However, in the holographic recording, optical phases can also be recorded as information, as well as the light intensity. In other words, there is an advantage that the hologram memory, in which information is recorded by holographic recording, can store intensity information and phase information. In addition, the hologram memory is different from the conventional optical memory, such as CD, DVD and BD, because the information is stored by effectively utilizing not only on the surface of the recording medium but also the volume of the recording medium. Therefore, there is another advantage that the hologram memory can store large-volume information. Further, in the holographic recording, the information is stored by utilizing the light intensity (amplitude) and phase. However, normally, either the intensity or the phase is used for recording the information in order to ease the handling of the recorded information.